Chapter 1: Introduction to Nanomaterials, Sol-gel Chemistryof Titanium oxide and Functional Applications
1.]. Overview on nanomaterials
Interest in the unique properties associated with materials having dimensions on
nanometer scale has been increasing at an exponential rate.“ ln nanoparticulate
materials, a large fraction of atoms is exposed on the surface of the particles. By
restricting ordered atomic arrangements to increasingly smaller sizes, materials begin to
be dominated by the atoms and molecules at the surfaces, often leading to properties that
are strikingly different from the bulk material. For instance, a relatively inert metal or
metal oxide may become a highly effective catalyst when manufactured as I1Ell1Op2ll'IlClCS,5
opaque particles may become transparent when composed of nanoparticles. or vice versa;
conductors may become insulators, or vice versa; and moreover the nanophase materials
may have many times the strength of the bulk material. Nanoparticles can comprise a
range of different morphologies including nanotubes, nanowires, nanofibres, nanodots
and a range of spherical or aggregated dendritie forms of different fractal dimensions.
These materials have seen application in a wide range of industries including electronics,
phamiaceuticals, chemical-mechanical polishing, materials for solid oxide fuel cells
(SOFCs), catalysis, and it is likely that the next few years will see a dramatic increase in
the industrial generation and use of nanoparticlcs. When the characteristic length scale of
the microstructure is in the 1-100 nm range, it becomes comparable with the critical
length scales of physical phenomena, resulting in the so-called "size and shape effects."
This leads to unique properties and the opportunity to use such nanostructured materials
in novel applications and devices. Phenomena occurring on this length scale are of
l
Chapter 1
interest to physicists, chemists, biologists, electrical and mechanical engineers, and
computer scientists, making research in nanotechnology a frontier activity in materials
science. Besides, the extremely high surface to volume ratio characterized by the
nanomaterials makes them highly reactive in terms of surface energy, which in tum let
the surface to undergo suitable reactions to reduce its surface energy. This possibility can
be exploited by using the nanomaterials in catalysis/photocatalysis.
1.2 Titanium Oxide
Titanium oxide has been known for many years as a constituent of naturally occurring
mineral ilmenite (FeO.TiO;) and belongs to the family of transition metal oxides. In the
beginning of the 20m century, industrial production started with titanium dioxide
replacing toxic lead oxides as pigments for white paints. Extraction of titanium oxide
from the mineral is a chemical process followed through a sulphate route or a chloride
r0ute.6 Many other processes such as plasma decomposition and direct reduction have
also been reported. Presently titanium oxide is well recognized as a valuable material
with application as a white pigment in paints, as filler in paper, textile and in
rubber/plastics} Titania has received a great deal of attention due to its chemical stability,
non-toxicity, low cost and other advantageous properties. While very high refractive
index (~2.4) and low visible absorptivity favour in the field of anti-reflection coatings
and in thin film optical devices, the wide band gap (~3.2 eV) combined with the high
ultraviolet absorption could be exploited in the field of optical coatings. Further, it finds
use in wastewater purification,8 inorganic membranes?’ '0 and as catalyst support. Titania
is a potential ceramic sensor element.“' '2 Titanium oxide is also being used in
heterogeneous catalysis, as a photocatalyst, in solar cells for the production of hydrogen
2
Chapter 1
and electric energy,'3"8 in ceramics, and in electric devices such as varistors. Titania has
excellent biocompatibility with respect to bone implants, a candidate material for gate
insulator in the new generation of MOSFETS, spacer material in magnetic spin-valve
systems, and also finds applications in nanostructured fonn in Li-based batterieslg and
electrochromic devices.”
Titania exists in three forms, rutile, anatase and brookite. Anatase (tetragonal, D4h'9
I41/amd, a=b=3.733 A, c=9.37 A), rutile (tetragonal, D41,"-P4;/nnmn, a=b=4.584 A,
c=2.953 A and brookite (rhombohedral, Dghls-PIJCEI, a=5 .436 A, b=9. 166 A ).2"22 Anatase
and rutile are in tetragonal structure and brookite is orthorhombic. In all three TiO2
structures, the stacking of the octahedra results in threefold coordinated oxygen atoms.”
Thennodynamically rutile structure is most stable. Brookite has an orthorhombic crystal
structure and spontaneously transforms to rutile at ~750 °C.24 Its mechanical properties
are very similar to those of rutile, but it is the least common of the three phases and is
rarely used commercially. In all the three crystalline forms each of the Ti“ ions are
surrounded by an irregular octahedron of oxide ions. Both in rutile and anatase the six
oxide ions that surround the Ti“ ions can be grouped into two. The two oxygen atoms are
farthest from Ti“ and the other four oxide ions are relatively closer to Ti4+. In rutile these
distances are 2.01A° and l.92A° respectively and in the anatase they are l.95A° and
l.9lA° (Figure 1.1). The anatase to rutile transformation is a metastable to
thermodynamically stable transformation and therefore there is no unique phase
transformation temperature as in the case of equilibrium reversible transformation.25
Anatase transforms irreversibly and exothermally to rutile in the temperature range 600
800 °C. The schematic diagram of unit cells for rutile and anatase is shown in Figure 1.1.
3
O0"l'i
e Q a
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Anatase
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Figure 1.1. Anatase and rutile unit cells and crystals
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Anatase has a tetragonal crystal structure in which the Ti-O octahedrals are connected by
their vertices as shown in Figure 1.1. Rutile has a crystal structure similar to that of
anatase, with the exception that the octahedrals are connected through the edges. This
4
Chapter 1
leads to the formation of chains, which are subsequently arranged in a four-fold
symmetry as shown in Figure 1.1. A comparison of layers in Figure 1.1 shows that the
rutile structure is more densely packed than anatase. As a point of reference, the densities
of the anatase and rutile phases are known to be 3.83 g/cm3 and 4.24 g/cm3 respectively.26
Typical properties of the major two crystal forms of titania are provided in Table 1.1.
Table 1.1. Typical properties of TiO;
Crystal form Anatase E RutileDensity (g/cm3)i 3.83 4.24Hardness (moh) I 5-6 l 6-7Crystal structure Tetragonal, 1 Tetragonal,Ul1if1Xifl1, negative Uniaxial, positiveCompressibility coefficient (106 em’ Kg") -- 0.53 - 0.58
Melting po1nt(°C) 3 *y [mm -- 1830115At higher % 02 " 1879 i 15
A Specific heat (Cal so‘ g‘) 0.17 0.17Dielectric constant 48 114
1.2.1 TiO; Photo catalysis
Photocatalytic applications of titania gained considerable emphasis in the 1990s
with the emerging demands on clean energy and protecting environment. Other oxides of
similar behaviour are zinc oxide, iron oxide, cadmium sulphide and zinc sulphide. Zinc
oxide is also a reasonable substitute for titania, except for its property of undergoing
incongruent dissolution resulting in formation of zinc hydroxide coating on the ZnO
5
Chapter I
particles which in turn leads to slow catalyst inactivation. Ideally, a semiconductor
photocatalyst should be chemically and biologically inert, photocatalytically stable, easy
to produce and use, efficiently activated by sunlight, able to efficiently catalyze reactions,
cheap and without risks to the environment or humans. Titanium dioxide (with sizes
ranging from clusters to colloids to powders and large single crystals) is close to being an
ideal photocatalyst, displaying almost all the above properties. The single exception is
that it does not absorb visible light. Both crystal structures, anatase and rutile, are
commonly used as photocatalyst, 27'“ with anatase showing a greater photocatalytic
activity 32’ 33 for most reactions. This increased photoreactivity is due to anatase’s slightly
higher Fermi level, lower capacity to adsorb oxygen and higher degree of hydroxylation
(i.e., number of hydroxy groups on the surface).34'36 Reactions in which both crystalline
phases have the same photoreactivity” or rutile have a higher one” are also reported.
Furthermore, there are also studies which claim that a mixture of anatase (70—75%) and
rutile (30—25%) is more active than pure anatase.394‘ The disagreement of the results may
lie in the intervening effect of various coexisting factors, such as specific surface area,
pore size distribution, crystal size, and preparation methods, or in the way the activity is
expressed. The behaviour of Degussa P25 commercial TiO2 photocatalyst, consisting of
an amorphous state together with a mixture of anatase and rutile in an approximate
proportion of 80/20, is for many reactions more active than both the pure crystalline
phases.42’43 The enhanced activity arises from the increased efficiency of the electron
hole separation due to the multiphase nature of the particles. Another commercial TiO;;
photocatalyst, Sachtlebem Hombikat UV 100, consisting only of anatase, has a high
photoreactivity due to fast interfacial electron-transfer rate.“ Main processes occurring
6
Chapter 1
on a semiconductor particle are: (a) electron—hole generation, (b) oxidation of donor (c)
reduction of acceptor and (d) electron~hole recombination at surface and in bulk,
respectively.” There are numerous photocatalytic reactions reported for titania.
Photocatalytic decomposition of trichloroethylene in water was investigated“ in which
anatase form was found to be better compared with rutile form. Titania prepared by sol
gel route was porous, having high specific surface area of ~ 600 m2g'l containing anatase
microcrystallites of the size of ~50 /it and was highly photoactive.“ Chloroform was
subjected to photo degradation in a medium containing suspended particles of titania.”
Similarly, phenol photo decomposition has been reported using fine titanium oxide.“
Photocatalytic reactions involving NO were conducted in presence of titania.” Silica as
support and titania as the active catalyst were tested for photo reactions and was
compared with the precursor characteristics.” Titania supported on alumina and silica
was used for photo catalytic decomposition of salicylic acid and found that the titania
alumina system showed improved performances‘ On analysis, it has been found that
titania-silica consisted of matrix isolated titania quantum particles while the TiO;-A1203
did not have such particles. Pt/Pd metal particle canying titania was also prepared and
tested. Titania film containing well dispersed Au or Ag metal particles were prepared by
sol-gel method, the effect of the dispersed metal particles on the photo-electrochemical
properties of the titania electrodes has been reported.” The photo responsive formation of
gold particles dispersed silica-titania composite gels were further investigated.” Photo
reduction of such systems containing Au(I1I) ions yielded gold particles and this principle
was used to produce micro pattems of gold particles on silica-titania films.
7
Chapter I
The titania sol-gel film coated on glass plate was exposed to water containing
bacteria and the sterilization rate was found to be increasing with increasing amount of
titania 54 and on the quantity of light absorbed by the titania thin film. Preparation and
characterization of semiconductor devices based on porous titania films and the
experimental result on photo conduction and trap states in titania have been reported.“
Dye sensitized titania film electrodes containing gold nano particles were investigated
and the results indicate that the UV photo response was lowered by the dispersion of gold
particles.56 The reason has been attributed to the shottky barriers at titania/gold interfaces
and the band edge fluctuation induced by the gold particles. The possibility of a
dissipative energy transfer from dyes to gold particles also has been indicated as a cause
for any particle associated titania. Performance was improved at slightly elevated
temperatures and a novel synergistic effect of photo and thermo catalytic behaviour has
been identified.” Thin films of titanium dioxide (TiO;) were deposited on variety of
substrates by a simple sol-gel dip coating technique from the titanium peroxide precursor
solution. The titanium oxide films were found to be very active for photocatalytic
decomposition of salicylic acid and methylene blue.58 Yoko et al. recently reported on the
Photo electrochemical properties of TiO; coating films prepared using different solvents
by the sol-gel method.” Chan et al. studied the effect of calcination on the
microstructures and photocatalytic properties of nanosized titanium dioxide powders
prepared by vapour hydrolysis.“ A homogeneous-precipitation route was adopted by Lee
et al.“ for the preparation of nanosize photocatalytic titania powders. Also, Watanabe et
al.62 reported on the photocatalytic activity of TiO; thin film under room light. Recent
Chapter I
reports indicate the improvement in the performance of nanosized titania photocatalysts
under sunlight excitation by using suitable dopants.“ 64
Table 1.2. TiO; compositions for photocatalysis
A Titania System Reaction system v!l
l TiO; films Trichloroethylene "TiO; aerogel = Aquatic decontamination D
if TiO2 suspension Chloroform 73“
TiO;; in zeolite structure ‘ if Phenol-MJ c_ __,_.
TiO2 in zeolite K Reactions of NOT-B
irioz/siozfriog/A1203 T “ Salicylic acid&phenol76
TiO; thin film Microbial sterilisation 5“
TiO2 nanofibrils Z Salicylic acid 77
A few other reports on lanthanum oxide doped titania include the work of Gopalan et al.°5
and LeDuc et al.66 There are reports on the effects of addition of metal ion dopants on
quantum efficiency of heterogeneous photocatalysis of titanium dioxide.“ The enhanced
photo activity of titania doped by rare-earth oxides such as Europium, Praseodymium and
Ytterbium oxides were reported by Ranjit et al.68 The high activity of oxide /TiO2 photo
catalysts is attributed to the enhanced electron density imparted to titania surface by the
dopnnt 0xideS_ A150, Lin gt n1_69 reported the effect of addition of YZO3, La2O3 and CeO2
on the photo catalytic activities of titania for the oxidation of acetone. The catalytic
property of V;O5/ La;O3-TiO; mixed oxide systems prepared by co-precipitation route
was reported by Reddy et al.7° The anatase form of titania is believed to possess enhanced
9
Chapter I
catalytic activity, probably due to its open structure compared to mtile and its high
specific surface area. Table 1.2 provides presence of various titania compositions and the
major chemical conversions reported for photocatalytic reactions.
1.2.2 Sol-gel synthesis of TiO;
The sol-gel process is a versatile solution process for making ceramic and glass
materials. In general, the sol-gel process involves the transition of a system from a liquid
sol into a solid gel phase. By applying the sol-gel process, it is possible to fabricate
ceramic or glass materials in a wide variety of forms: ultra fine or spherical shaped
powders, thin film coatings, ceramic fibres, microporous inorganic membranes,
monolithic ceramics and glasses or extremely porous aerogel materials.78'82 An overview
of the sol-gel process is presented in Figure 2.2.
TiO2 nanomaterials were synthesized by sol-gel method from hydrolysis of titanium
precusor. These methods are used for the synthesis of thin films, powders, and
membranes. Two types are known: the non-alkoxide and the alkoxide route. Depending
on the synthetic approach used, oxides with different physical and chemical properties
may be obtained. The sol-gel method has many advantages over other fabrication
techniques such as purity, homogeneity, felicity, and flexibility in introducing dopants in
large concentrations, stoichiometry control, ease of processing, control over the
composition, and the ability to coat large and complex areas.
The non-alkoxide route uses inorganic salts “'85 such as nitrates, chlorides,
acetates, carbonates and acetylacetonates, which require removal of the inorganic anion,
while the alkoxide route (the most employed) uses metal alkoxides as starting material.86'
88 This method involves the formation of a TiO; sol or gel or precipitate by hydrolysis and
10
Chapter I
condensation (with polymer fomtation) of titanium alkoxides. This process normally
proceeds via an acid-catalyzed hydrolysis step of titanium (IV) alkoxide followed by
condensation.” 90 The development of Ti-O-Ti chains is favoured with low content of
water, low hydrolysis rates, and excess titanium alkoxide in the reaction mixture.
nmgefflm Dense film-.5-t‘ *“.-4-P""-F-FF‘-' 9;'i"qP :II’.l_;L' 0'5.do 0 9 I
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Three dimensional polymeric skeletons with close packing result from the development
of Ti-O-Ti chains. The formation of Ti(OH)4 is favoured with high hydrolysis rates in the
presence of medium amount of water. The presence of a large quantity of Ti-OH and
insufficient development of three-dimensional polymeric skeletons lead to loosely packed
first-order particles. Polymeric Ti-O-Ti chains are developed in the presence of a large
excess of water. Closely packed first order particles are yielded via a three-dimensionally
ll
Chapter I
developed gel skeleton.9"'°2 From the study on the growth kinetics of TiO; nanoparticles
in aqueous solution using titanium tetraisopropoxide (TTIP) as precursor, it is found that
the rate constant for coarsening increases with temperature due to the temperature
dependence of the viscosity of the solution and the equilibrium solubility of TiO;_
Secondary particles are formed by epitaxial self-assembly of primary particles at longer
times and higher temperatures, and the number of primary particles per secondary particle
increases with time. The average TiO2 nanoparticle radius increases linearly with time, in
agreement with the Lifshitz-Slyozov- Wagner model for coarsening. In order to exhibit
better control over the evolution of the microstructure, it is desirable to manipulate the
steps of hydrolysis and condensation.l°3 In order to achieve this goal, several approaches
were adopted. One of them is alkoxide modification by complexation with coordination
agents such as carboxylates'°4'l°9 or diketonates that hydrolyze slower than alkoxide
ligands. Additionally, the preferred coordination mode of these ligands can be exploited
to control the evolution of the structure. In general, [3-diketonello ligands predominately
form metal chelatesl '1 which can cap the surface of the SlII'UCtUI‘C.H2 Carboxylate ligands
have a strong tendency to bridge metal centersm which are likely to be trapped in the
bulk of materials and on the surface of the particle."4 Acid-base catalysis can also be
used to enable separation of hydrolysis and condensation steps.” It has been
demonstrated that acid catalysis increases hydrolysis rates and ultimately crystalline
powders are formed from fully hydrolyzed precursors. Base catalysis is thought to
promote condensation with the result that amorphous powders are obtained containing
unhydrolyzed alkoxide ligands. On the other hand, acetic acid may be used in order to
initiate hydrolysis via an esterification reaction, and alcoholic sols prepared from titanium
12
Chapter I
alkoxide using amino alcohols have been shown to stabilize the sol, reducing or
preventing the condensation and the precipitation of titania.“ These reactions are
followed by a thermal treatment (450—600 °C) to remove the organic part and to
crystallize either anatase or rutile TiO2. Recent variants of the sol—gel method lowered
the necessary temperature to less than 100 °C.“7 The calcination process will inevitably
cause a decline in surface area (due to sintering and crystal growth), loss of surface
hydroxyl groups, and even induce phase transformation. Washing steps have also been
reported to cause surface modifications."8’ H9 Cleaning of particles is usually achieved by
washing the surface with a solvent, followed by centrifugation. The solvent can affect the
chemical composition and crystallization. It was also reported that particle washing could
affect the surface charge of the particles by bonding onto the surface. An alternative
washing technique is to dialyze particles against double-distilled water,'2° which could be
an effective method of removing soluble impurities without introducing new species. As
titanium sources, Ti(O-Et)4,'2' Ti(i-OP)4m"24 and Ti(O-nBu)4l25‘m are most commonly
used. The sol—gel method has been widely studied particularly for multicomponent
oxides where intimate mixing is required to form a homogeneous phase at the molecular
level. Thus, metal ions such as Ca2+,'28 Sr” ,Ba2+ ,Cu2+,'29"3° Fe3+,m"34 V5+,'35 Cr”,
Mn2+, Pt4+,136 c02+,131 Ni2+, Pb2+,1ss W6+’ Zn2+,l39 Ag: Au3+,14o.141 Z1_2+’142 La3+,|43 and
Eu“ were introduced into TiO2 powders and films by this method and the photocatalytic
activity was improved to varying extent. Most nanocrystalline—TiO2 (nc-TiO2) particles
that are commercially obtainable are synthesized using sol—gel methods. Very recently,
sol—gel and templating synthetic methods were applied to prepare very large surface area
titania phases'44"46 which exhibit a mesoporous structure. Ionic and neutral surfactants
13
Chapter I
have been successfully employed as templates to prepare mesoporous TiO2.'47"52 Block
copolymers can also be used as templates to direct formation of mesoporous TiO2.l5 3'15 5
In addition, many non-surfactant organic compounds have been used as pore formers
such as diolates'56"57 and glycerine.158’ '59
Sol—gel methods coupled with hydrothermal routes for mesoporous structuresléo
lead to large surface area even after heating at temperatures up to 500 °C. This may be
explained as follows: generally, mesopores collapse during calcination due to
crystallization of the wall. When a hydrothermal treatment induces the crystallization of
amorphous powders, the obtained powders can effectively sustain the local strain during
calcination and prevent the mesopores from collapsing. For nanostructured thin films, the
sols are often treated in an autoclave to allow controlled growth of the particles until they
reach the desired size. Oswald ripening takes place during this process, leading to a
homogeneous particle-size distribution. If a film is made using these particles, substances
can be added to prevent cracking and agglomeration or increase the binding and viscosity
after this ripening process. The resulting paste can be deposited on a substrate using
doctor blading or screen printing. The solvent is evaporated and the particles are
interconnected by a sintering process, normally at air temperatures around 450 °C. At this
temperature, organic additives are also removed from the film. Slow heating and cooling
is important to prevent cracking of the film. In most cases, the resulting film has a
porosity of 50%. Thin films can also be made from the sol by dip coating.
14
C hapter I
1.2.3 Anatase - Rutile Phase Transformation
Anatase and rutile are the two polymorphs of titania at atmospheric pressure. The
room temperature phase is anatase and the high temperature phase is rutile. Anatase
transfomis irreversibly and exothermically to rutile in the range 400 °C to 1200 °C'6" '62
depending on parameters such as the method of preparation, grain size, morphology,
degree of agglomeration, nature of impurities and reaction atmosphere.'63"66 At
atmospheric pressure the transformation is time and temperature dependent and is also a
function of impurity concentration. The complexity of the transition is typically attributed
to the reconstructing nature. The transition is a nucleation-growth process and follows the
first order rate law with activation energy of ~90 kcal/mo1.l67
The anatase - rutile transfomiation involves an overall contraction of oxygen and
movement of ions so that a cooperative rearrangement of Ti“ and 02' occur. The
transformation implies that two of the six Ti-O bonds of anatase structure break to form a
rutile structure. The removal of the oxygen ions, which generate lattice vacancies,
accelerate the transformation and inhibit the formation of interstitial titanium. The
impurities that have most pronounce inhibiting action are chloride, sulphate and fluoride
ions whereas that accelerates the transformation includes alkaline and some of the
transition metal ions. Those ions with valency greater than four reduce the oxygen
vacancy concentration and will retard the reaction.l68
The effect of reaction atmosphere shows that vacuum conditions and atmosphere
of hydrogen, static air, flowing air, oxygen, argon, nitrogen and chlorine affect the phase
transfonnation to different extents. Lida and Ozaka as well as Shannon found that the
transformation rate in a hydrogen atmo.sphere is greater than in air and under vacuum the
15
C hapler I
rate of transformation decreases as oxygen partial pressure increases.'69 Oxygen
vacancies are formed in hydrogen atmosphere whereas the interstitial Ti3+ ions are
generated under vacuum. The rate constant of the transformation in hydrogen was 10
times larger than in air.'7° It has been reported that at 950 °C the phase transformation in
Ar/Cl; atmosphere is about 300 times faster than in air.m The accelerating effect of
chlorine atmosphere on the anatase-rutile phase transformation involves two mechanisms
that probably occur simultaneously - vapour mass transport and oxygen vacancy
formation in which the first generate nucleation and growth in the bulk. When the vapour
transport is negligible, the primary mechanism is based on oxygen vacancies.
The effect of metal cations such as Li, Na, K, Mg, Ca, Sr, Ba, Al, Y, La, Er, Co,
Ni, Cu and Zn on anatase - rutile transformation was studied earlienm A linear
relationship between phase transition temperature and ionic radius, for alkali and alkaline
earth metals and group III elements are reported. Transition metals, which entered the
matrix interstitially, gave a high transition temperature, whereas those dopants introduced
substitutionally did not give a significant change in transition temperature. It was
concluded that the oxidation state together with ionic radii of cations and type of sites
' occupied were the important parameters, which control phase transition temperature.
Depending on the ionic radius of dopant compared with radius of titanium, it can be
introduced substitutionally or interstitially or if the size of dopant is larger than oxygen, it
could be intercalated into the matrix, producing a lattice deformation. From that study
dopant appears to have no effect on the amorphous gel to anatase transformation
temperature, but influenced the anatase - rutile transformation. If dopant ion size is less
than that of titanium, anatase phase will be stabilized to a higher temperature. Dopants
I6
Chapter I
bigger than oxygen ion produce large local deformation of lattice. Those dopant ions
whose size fall in between titanium and oxygen stabilize the anatase phase. Those
dopants near to oxygen size can stabilize the titania phase more. The enhancement or
inhibiting effect of additives on anatase - rutile transformation depends on their ability to
enter the TiO2 lattice, thereby creating oxygen vacancies or interstitial Ti3+ ions. Oxides
of Cu, Co, Ni, Mn and Fe mixed with anatase TiO2 increases the transfonnation rate
efficiently. Transition metals, which entered the matrix interstitially, gave a high
transition temperature, whereas those dopants introduced substitutionally did not give a
significant change in transition temperature. m‘ 173
Bacsam reported an improvement of the anatase-to-rutile phase transformation by
peptizing the hydrolyzed precipitates with nitric acid, however, the l00% rutile phase was
not obtained. Bischoff '75 and Anderson found that acid peptization of TiO; particles
favoured the formation of rutile, in comparison with the situation that occurred at higher
temperatures. It is generally accepted that the adsorption of protons on the surface of
hydrous TiO; particles creates a net positive charge, and thus yields an electrostatically
stabilized sol during acid peptization. However, this adsorption model of peptization
could not explain the rutile phase formation after peptization at low temperature. Zhang
et al.'76 used hydrochloric acid as peptizing agent and the phase formation of
nanoparticles during the antiaggregation process was attributed to its chloride ion.
Ferreira reported the effect of inorganic acid and base concentration on the anatase to
rutile phase transformation and proposed a reaction mechanism for rutile formation. It is
interesting to note that an increased concentration of electrolyte enhanced the rutile
fomiation and the effect was shown even at room temperaturem
l7
Chapter 1
1.2.4 High temperature catalysts
Most of the applications of titania ceramics at high temperature calls for the pure
rutile phase which is usually formed by heating titanium salts above 600 °C. However,
with the expanding applications in the area of catalysts, photo catalysts, membranes and
active humidity sensors, the need for obtaining anatase phase stable at elevated
temperatures become significant. Earlier work indicates that even as a surface modifier
for anatase titania pigments, alumina was used as a coating in order to improve gloss
property as well as to prevent degradation. Recent identification of ‘self-cleaning’
surfaces by transparent anatase coatings on glass, ceramic tiles and bricks,178 the anatase
phase has to be retained at the processing temperature above 1000 °C. The anatase-rutile
transformation temperatures are fairly dependent on the history of the sample.l79' 173
Further, the low temperature densification in titania could be associated with the phase
formation temperature. Early indicative reports on the incorporation of aluminium oxide,
copper oxide, manganese oxide, iron oxide and zinc oxide postulated that the mechanism
for modification of anatase-rutile transformation is related to oxygen vacancies on titania.
This was also explained that the dispersion of alumina on titania stabilizes its surface and
increases the apparent activation energy for the rutile nucleation at titania-alumina
interfaces. By using copper chloride as a dopant solution, a modified titania having
nanocrystalline brookite stable at 400 °C and having a narrow band gap than normal
titania, could be produced through sol-gel route.'8° However, a detailed investigation
using thermal analysis and XRD techniques on the role of alumina in increasing the
anatase-rutile transformation indicatem that a metastable anatase solid solution
containing alumina is formed at relatively low temperatures, and alumina is formed from
18
Chapter I
exsolution process of the as formed anatase solid solution, in which rutile is formed at
higher temperature. This argument is further supported by the fact that oc-alumina is
formed at as early as 900 °C in presence of titania while the usual oc-alumina formation is
above 1100 °C. The influence of addition of zirconia in the raising of transformation
temperature of anatase to rutile is also reported. Since zirconia is not expected to involve
in any oxygen vacancy change in titania, the role of zirconia was identified to be due to
incorporation of Zr ions into anatase lattice. The formation of a limited solid solution of
zirconia in anatase at low temperature increased the strain energy and thus leads to a
higher anatase to rutile transformation temperature.'82 An investigation on the effect of
several cations of lanthanum, zinc, aluminum, potassium, sodium, calcium, barium and
cobalt on the anatase-rutile transfomiation has been reported.‘83 The dopants were
introduced into the titania gel in the form of nitrates, heat treated in the range 350-1100
°C and was characterized by wide angle X-ray diffraction (WAXS) and
thermogravimetry. Lanthanum oxide was doped in titania membrane precursors in order
to study the thermal stability and it was seen that there was an increase of 150 °C in the
anatase to rutile transformation in the doped composition.'84 SnO;_ A1203, and Fe2O3 were
doped in nanocrystalline titania precursors and found that while SnO2 and Fe;O3 decrease
the transformation temperature, A1203 increased the same. However, the interesting fact
is that these oxides were successful in controlling grain growth, which normally occurs in
rutile as a result of the transformation. As is known in the case of nanocrystalline
materials, the grain growth can be regarded as coalescence of smaller neighbouring
grains, where grain boundary motion is mainly involved, and the role of these dopant
oxides would be to restrict the movement of these grain boundaries thus lowering the
19
Chapter I
grain growth.'85 The transformation kinetics in presence of Fe2O3 has been reported,'86
where Fe2O3-TiO2 mixture was heated in air and in argon atmosphere to different
temperatures and the phases formed were analyzed by using XRD techniques. As found
in the earlier study, the Fe;O3 primarily decreases the anatase to rutile transformation
temperature.
Platinum was incorporated in titania prepared through titanium butoxide and
platinum acetyl acetonate.'87 Platinum promoted the formation of rutile probably through
metal catalyzed dehydroxylation of anatase precursor or through the presence of PtO;
which has the rutile structure, as an intermediate phase. Platinum atoms, however, did not
go into crystalline structure of rutile. In another study, chromium (III) was incorporated
in anatase titania catalyst in different concentrations and analysis of the cell parameters
indicated that there is a stability limit for the system at ~1.4 atomic percentage.
Acceleration in the rate of anatase to rutile phase transition was also reported.'88 Further,
nanosize silver was incorporated in titania precursor gel and its effect of A>R
transformation was investigated using impedance spectral measurements. The
transformation was delayed in presence of si1ver.'89
1.2.5 Titania Functional Coatings
The concept of development of ‘self-cleaning’ surfaces was reported in the
ninentees,]90 which was a step further on the application of photo responsive behaviour of
titanium oxide. They prepared a thin TiO2 polycrystalline film from anatase sol on a glass
substrate which on UV irradiation, the contact angle of the surface decreased to 0 i 1°
from that of 72 i 1°. They found that irradiation created a surface that was highly
hydrophilic and oleophilic. This was due to the creation of surface oxygen vacancies at
20
Chapter I
bridging sites on UV irradiation, which resulted in conversion of Ti“ sites to Ti3+ sites
that favoured the dissociative adsorption of water molecules and also influenced the
affinity to chemisorbed water of its surrounding sites. This increase in surface wettability
due to the formation of functional groups such ashydroxyl groups that is increased by the
irradiation of light.'9"'94 A drop of water falling on a surface spreads very uniformly and
therefore provides an even surface and excellent transparency. Super hydrophilic surfaces
also provide antifogging property.l95 However, organic additives, which usually are
responsible for this function, have low stability with respect to mechanical, thermal and
environmental considerations. Titania is a potential candidate in this line in view of their
availability, stability and possibility to prepare in the form of nano c0atings.l96 Thus, a
successful self-cleaning property is associated with synergic effect of photo catalytic
decomposition of compounds and also by hydrophilicity, by which drops of water spread
out evenly and clean the surface by removing decomposition products. These
combination surfaces will have wide applications on windows of high rise buildings,
optical glass, automobile window shields and rear view mirrors, removal of oil smears
from surfaces when immersed in water, self cleaning of kitchen exhaust fans and floors of
public comfort stations and hospitals.14’l97’I98 Contaminants on exterior walls of buildings
can be washed by rain water much more efficiently or can be cleaned easily by jets of
water. Sol-gel derived mesoporous titania films are also reported for applications in
catalytic nano and ultra filtration membranes required in technologies such as gas
separation, catalysis, membrane reactors, sensors and adsorbents. Sol-gel technique is a
very good means to control the porosities of both bulk and thin film materials.-'99
Recently, the use of organic or microporous templates is catching up in the process of
21
Chapter I
porosity control, besides the more traditional particle packing approach to prepare
controlled porosity materials. Titanium oxide having macropores to micro pores and
nanopores have been investigated 20° for drawing conclusions on preparation parameters
and correlation to end properties, with considerable success.
22
Chapter 1
1.3 Definition of the present problem
Titanium oxide is used in heterogeneous catalysis and as a photocatalyst for the
decomposition of organics, in the treatment of industrial waste water, for elimination of
harmful bacteria and in the photocleavage of water, in solar cells for the production of
hydrogen and electric energy and in antifogging and self cleaning coatings. Even though
lots of studies are reported on the synthesis and on various properties of titania, sol-gel
method is shown to be an effective route for the synthesis of nanocrystalline titanium
oxide powders. Bulk of the sol-gel synthesis and property evaluation are reported on
titania derived from alkoxide precursors. Even though the method is well investigated,
the commercialization aspect of various technologies using titania is not addressed well
when alkoxide precursors are used. The much abundant industrial source of titania is still
the metal salts. Hydrolysis condensation reactions are faster for the metal salts compared
to the alkoxide and hence the control of the sol-gel reaction along with its application
becomes difficult. So there is a need for development of a sol-gel process using the
cheaper salt precursors. The present thesis develops from this point of view of titania sol
gel chemistry and an attempt is made to address the modification of the process for better
photoactive titania by selective doping and also demonstration of utilization of the
process for the preparation of supported ceramic membranes. Therefore, in the present
work an attempt is made to
l. Study the synthesis of nanocrystalline titania using an aqueous sol-gel
method starting from titanyl sulphate and optimising process parameters.
2. Modify the textural properties of titania by selective doping (Ta5+, Gd“
and Yb3+) using tantalum oxalate, gadolinium nitrate, ytterbium nitrate.
23
Chapter 1
3. Characterize the powder for anatase to rutile phase transformation,
crystallite size, specific surface area, catalytic and photocatalytic
properties. Correlation of synthetic procedure and properties of
photocatalytic titanium oxide.
4. Fabrication and detailed morphological investigation of titania membrane
on porous alumina substrates and filtration studies.
5. Photoactive nanocrystalline titania coatings on glass surfaces for possible
self cleaning applications.
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